Stringy filament - a temperature thing?

For the last 12 months or so, I have been using PLA filament with a diameter of 1.75mm on a Huxley machine and never noticed or had any problems with the production of long stringy filament strands which I now seem to get when using 3mm filament from the same source on a bigger machine. Is this likely to be related to temperature? The problem can be quite annoying - if the nozzle sits above the bed and starts to ooze plastic, grabbing hold of it and pulling it away can take ages as the filament turns into a never ending string which is very difficult to see, (and if you are not careful, prone to wrap itself around the Z axis lead screws!).

On a similar subject, how do you know when you have the right temperature for a particular filament? Too cold, much too cold and I can see that hand turning the extruder gear will require force but too hot? I have never experienced filament dripping out of the nozzle like syrup and if that happened, I guess that I could recognise it but how to find the optimum temperature is somewhat difficult (for me that is).

Quoting temperature figures and working with those reported by others is prone to problems - just because Pronterface says the filament is at 200degrees (or whatever) doesn't mean that it actually IS at 200degrees for a whole host of reasons so that is a no-go solution I think. Is there a technique, a thing to watch out for which would help? After a few blocked JHead experiences, I am a wee bit frightened of going too low yet don't want to over compensate with temperatures which are too high. I have a number of different colours of filament and feel that some need more heat than others despite them all being PLA.
All advice gratefully received.

Alan

Edited 1 time(s). Last edit at 12/20/2012 02:34PM by Alzibiff.

What I did is start at a high temperature and reduce it in steps of 5 degrees untill the extruder couldn't extrude any more. I then added 10 degrees to this and this is what I use. I normal extrude the first layer 10 degrees higher again. What slicing software do you use? Can you configure some retraction.

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Using ABSPrusa Mendel Zaphod with Pronterface and slic3r 1.3.0. Printing well with 3mm PLA and ABS through 2 x J Head Mk IV b and Wade Geared Exruders. Controlled using RAMPS1.4 board running Marlin_v1.1.9

Hi there! I'd love to help you isolate this problem.
I believe it has to do directly with the extruder head temperature. If you can provide me some more information i'm sure we can nail it down. Based off your information, here's some ideas;

PLA starts liquifying at around 185C. At 200C, it definitely seeps and drips out of the nozzle as the force of gravity is too much for it to bear! If your other machine did not have the seeping problem there can be a couple of answers:
Your machines extruder head was actually not at 200C.
Your smaller machine had a smaller liquification chamber. If both machines share the same nozzle diameter, the "larger" machine that accepts the 3.0mm diameter spool might have a larger liquification chamber. The oozing/dripping is directly related to the PLA's surface tension at a specific temperature.

You're dealing with hydraulic flow resistance of a liquid grabbing a specific material ie; steel/copper (extruder head material.) This is similar to how a drop of water starts small then gets larger to a certain point before "dripping. If you had a container (liquification chamber) with a larger amount of liquid, the amount of "drips" that would occur over the same interval would be greater in the system with a larger liquid source.

Not all PLA melts at the temperatures you say and due to the time it takes for the heat to transfer to the plastic you do have to print at high temperatures sometimes even with PLA. Ultimakers print PLA at 220c and I print from 160c - 220c on Tantillus depending on the filament, layer size and speed.

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